1. Field of the Invention
The present invention relates to a transfer unit, a transfer method, and an image forming apparatus, which adopt an intermediate transfer method in which an intermediate transfer member is used and a toner image is transferred from the intermediate transfer member to a sheet.
2. Description of the Related Art
A color image forming apparatus is provided with a transfer unit that transfers a color toner image formed by an image forming section onto a sheet. A known transfer method adopted by conventional transfer units is an intermediate transfer method in which (i) color toner images are formed by image forming sections that produce different colors, and are sequentially transferred onto an intermediate transfer member such that the color toner images are overlaid on top of each other (primary transfer), and then (ii) the overlaid color toner images are simultaneously transferred onto a sheet in one transfer step (secondary transfer). In the intermediate transfer method, the thickness of the overlaid color toner images formed on the intermediate transfer member is thicker than that of a monochrome toner image. As such, it is effective to utilize a transfer belt, in addition to the intermediate transfer member, as a secondary transfer member for transferring a toner image onto a sheet. This increases the transfer nip, and therefore the color toner image is transferred onto a sheet more efficiently.
In this manner, the transfer efficiency of the toner image onto a sheet can be improved by the secondary transfer method utilizing a transfer belt. This, however, causes not only the toner image but also “toner-fog”, which is originated from a residual electric potential, to be transferred onto the transfer belt. Consequently, the transfer belt is contaminated. Therefore, a cleaning mechanism for the transfer belt will be necessary.
Generally, a mechanical mechanism utilizing a cleaning blade is most always adopted as the cleaning mechanism. However, if the transfer belt to be cleaned is thin and soft, then it is not desirable to bring a hard blade into contact with such a transfer belt because this may cause various problems. For example, wrinkles may be generated on the belt, the belt may wobble, or the belt may be damaged. Such problems may arise not only in the case where the transfer belt is used as the secondary transfer member, but also in a case where a soft transfer roller is utilized so as to increase the transfer nip.
As a countermeasure, for the cleaning of the secondary transfer member (e.g., the transfer belt, the transfer roller), a method is commonly adopted in which an electric field is applied to the secondary transfer member, which electric field has the opposite polarity to that of an electric field for transferring a toner image from the intermediate transfer member onto a sheet, so that unnecessary toner adhered to the secondary transfer member is transferred back to the intermediate transfer member. Such art and related art are taught in Japanese Unexamined Patent Publications, No. 112267/2000 (Tokukai 2000-112267) (published on Apr. 21, 2000) and No. 161051/1999 (Tokukaihei 11-161051) (published on Jun. 18, 1999).
However, not all toners adhered to the secondary transfer member (e.g., the transfer belt, the transfer roller) are normally-charged toners of a desired polarity, and some of the toners may be oppositely-charged or not charged at all. Therefore, if an electric field having the opposite polarity to that of the electric field for transferring is applied to the secondary transfer member, the normally-charged toner is transferred back to the intermediate transfer member but the oppositely-charged toner and uncharged toner remain on the secondary transfer member. This causes a problem that the oppositely-charged toner and the uncharged toner generate fog on a sheet in a subsequent transfer step (for example, when the transfer step is performed on a sheet of a larger size).